One important measure of the quality of a video display, including flat panel displays and cinema projection screens, is the uniformity of brightness, or luminance. With respect to cinema projection screens, in order to maintain an optimal viewing experience, cinema operators must periodically perform a measurement of the consistency of luminance across the screen in both the horizontal and vertical dimensions. Both the cinema screen itself and the video projection system can contribute to luminance non-uniformity. Over time, a screen can become dirty or damaged, creating the appearance of “dark” or “light” spots on the screen. These dirty or damaged spots can be colored so as to only affect certain colors of light, thereby creating a color-cast across the screen. Even on a clean and well-maintained screen, the distance of the projector from the screen and the type of lens used on the projector can cause areas of illumination non-uniformity. Additionally, a cinema's ambient lighting may create areas of inconsistent screen illumination.
Various industry organizations prescribe standards for measuring a cinema screen's luminance uniformity. Both the American National Standards Institute (ANSI) and the Society of Motion Picture and Television Engineers (SMPTE) standards specify measuring a screen's brightness in each of nine zones, wherein each zone is defined by dividing the screen into a 3×3 grid consisting of nine equally sized rectangles. For example, see SMPTE RP98-1995 (hereinafter “the relevant measurement standard,” “the relevant SMPTE standard,” or simply “the relevant standard”), which states that “the screen is considered to be divided into nine equal-size rectangles.” The standard further specifies that a luminance measurement for each of these nine zones is to be made from six different measurement locations within the theater: the left edge seat, right edge seat, and center seat of the center row of the theater, and the left edge seat, right edge seat, and center seat of the rear row of the theater. At each measurement location (seat) within the theater, the nine measured luminance values are recorded and the screen is determined to be within specification if the measured luminance of center zone of the screen is within specified absolute limits (e.g. “between 12 fL and 22 fL”), and if the measured luminance of the remaining 8 zones of the screen are within specified limits relative to the center zone (e.g. “luminance of the screen sides and corners shall be at least 80% of the screen center reading”), as well as relative to each other (e.g. “the remaining eight measurement points shall not differ by more than 4 fL”).
Each measurement is typically made using a spot light meter, such as the Spectra Cinespot One-Degree Spotmeter (see http://www.spectracine.com/Product_3.html), by displaying a flat field (all white) frame on the screen, then sequentially pointing the spot light meter at the center of each of the nine zones of the screen and recording the light value measured by the meter. Because these types of spot light meters have narrow view-angle lenses, the meter only “sees” a small area of the screen, and thus measures the reflected light only from the particular zone being measured. A spot light meter suitable to make such a measurement can be significantly expensive.
An alternative to using a spot light meter would be to use a simple, lower-cost, wide-angle reflected light meter or wide-angle incident light meter. Because a wide-angle meter “sees” the entire screen, and not just the particular zone being measured, a wide-angle meter would need to be used in conjunction with a sequence of patterns that could be displayed on the screen, where each pattern illuminated only one of the nine measurement zones. However, the wide-angle meter would be required to have a high dynamic range to be capable of measuring these relatively low light levels (one-ninth of full screen brightness). A high dynamic range would make the required wide-angle light meter more expensive. Also, the ambient light in the theater would need to be substantially reduced in order to avoid contaminating these low light level measurements. Reducing the ambient light to an acceptable level could be impractical or inconvenient in many cases.
Furthermore, regardless of whether a spot light meter or a wide-angle meter is used, the measurement process described above requires at least 54 separate manual measurements (1 measurement per measurement zone times 9 zones per measurement location times 6 measurement locations), which can be tedious and prone to error.
What is needed is a method and apparatus of measuring a screen's brightness uniformity that does not require reduction of the ambient light in the theater, and that allows the use of a simple, wide-angle light meter without a large dynamic range, such as a typical photocell light meter, or a photocell incorporated into a typical modern mobile phone's camera.